Method for making carbon nanotubes

ABSTRACT

A method for making carbon nanotubes that includes the following steps. A metal substrate is provided. The surface of the metal substrate is polished. The polished metal substrate is put into a reaction device. A protecting gas is introduced to the reaction device while the environment inside of the reaction device is heated to about 400 to 800 degrees. A mixture of carbon source gas and protecting gas is introduced to the reaction device, whereby the carbon nanotubes are grown on the metal substrate on the polished metal substrate.

BACKGROUND

1. Technical Field

The present disclosure relates to methods for making carbon nanotubes and, particularly, to a method for making carbon nanotubes on a metal substrate.

2. Discussion of Related Art

Carbon nanotubes (CNTs) are a novel carbonaceous material discovered by Iijima, a researcher of NEC Corporation, in 1991. Typically, carbon nanotubes have tube-shaped structures with small diameters (less than 100 nanometers) and large aspect ratios (length/diameter). They have excellent electrical properties as well as excellent mechanical properties. The electronic conductance of carbon nanotubes is related to their structures. Because the carbon nanotubes can transmit extremely high electrical current and emit electrons easily, at less than 100 volts, they are considered to be promising for use in a variety of electrical devices.

Generally, a number of electronic devices, such as field emission devices, traveling-wave tubes or electron guns, employ the carbon nanotubes as electron emitters. In order to achieve high power requirements, a substrate for supporting carbon nanotubes should have an ability to endure large amounts of electrical current to pass through. Therefore, it is understood that a substrate made of metal with high conductivity is considered to be a good option for use.

Currently, a method of chemical vapor deposition (CVD) is mainly adopted for forming the carbon nanotubes on the substrate. CVD is performed by coating metal catalysts, such as transition metal or transition metal complex, on the substrate and directly synthesizing the carbon nanotubes on the substrate. In principle, a carbon source gas is thermally decomposed at a predetermined temperature in the presence of the metal catalyst, thereby forming the carbon nanotubes.

However, once the transition metal is used as a catalyst and coated on the metal substrate, it is easy for the transition metal reacting on the metal of the metal substrate to form an alloy. Thus, the transition metal has become an inactive catalyst, and the catalytic reaction for growing carbon nanotubes will be affected. What is needed, therefore, is to provide a method for making carbon nanotubes, which is able to be performed easily on a metal substrate and is suitable to be employed in mass production.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present method for making carbon nanotubes can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present method for making carbon nanotubes.

FIG. 1 is a flowchart of a method for making carbon nanotubes, in accordance with a present embodiment.

FIG. 2 is a scanning electron microscope (SEM) image of carbon nanotubes formed using the method in accordance with the present embodiment.

FIG. 3 is a transmission electron microscopy (TEM) image of carbon nanotubes formed using the method in accordance with the present embodiment.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one embodiment of the present method for making carbon nanotubes, in at least one form, and such exemplifications are not to be construed as limiting the scope of the disclosure in any manner.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Reference will now be made to the drawings to describe, in detail, embodiments of the present method for making carbon nanotubes. Referring to FIG. 1, a method for making carbon nanotubes, according to a present embodiment, includes the following steps:

Step 1, providing a metal substrate, S1. In the present embodiment, the metal substrate is a copper substrate. The metal substrate can vary in shape and thickness according to practical requirements. For example, the metal substrate can be a solid rectangular piece. A thickness of the metal substrate can be in a range of about 0.5 centimeters (cm) to about 5 centimeters. An area of the metal substrate can be in a range of about 4 cm² to 100 cm².

Step 2, polishing a surface of the metal substrate, S2, is detailed below. In the present embodiment, the surface of the metal substrate is rubbed along a first direction with an abrasive paper for about 3-5 minutes. The abrasive paper is about 600-800 grit. After that, powder generated by the sanding during the polishing process is removed by an application of air flow, e.g. blowing. The treated surface of the metal substrate is then rubbed along a second direction with an abrasive paper for about 5-8 minutes. The abrasive paper employed in such step is about 1000-1300 grit. The powder generated due to sanding in this step is also removed. After rubbing the surface of the metal substrate along the second direction, the surface of the metal substrate is rubbed along the first direction with an abrasive paper for about 10-15 minutes. In this case, the abrasive paper is about 1500-2000 grit. Finally, powder generated by this is removed. An angle α between the first direction and the second direction is in a range of 0°<α≦90°. Particularly, in the present embodiment, the angle α is about 90°.

As a result, the surface of the metal substrate is substantially flat and smooth by way of the polishing in step 2 that will facilitate the growth of the carbon nanotubes on the metal substrate. However, it is understood that there are micro variations in the form of notches or fine grooves that can be observed on the surface of the metal substrate. Particularly, such the notches or fine grooves are formed on a nanometer scale and in a net-like pattern due to repeatedly rubbing steps.

Step 3, putting the polished metal substrate into a reaction device, S3. In the present embodiment, the reaction device is a furnace, e.g. a box furnace or a tube furnace. Particularly, the polished metal substrate is put into a quartz boat, which is subsequently inserted into the center of the tube furnace.

Step 4, introducing a first protecting gas while heating the environment inside of the reaction device, S4. The first protecting gas can be nitrogen. In the present embodiment, the environment inside of the reaction device is heated to about 400-800 degrees (C). For example, the environment inside of the reaction device is heated to about 700 C. In step 4, during the heating process, a plurality of metal particles, e.g. copper particles, forms around the notches or fine grooves formed on the surface of the metal substrate, and can serve as seeds for facilitating the growth of carbon nanotubes. In the present embodiment, diameters of the metal particles range from about 1-10 nanometers (nm). In addition, the density of the metal particles is closely related to the number of times of rubbing and the angle of the rubbing directions between different rubbing steps. It is understood that the higher density of metal particles is obtained by the greater number of times of rubbing and the smaller angle of rubbing directions between different rubbing steps.

Step 5, introducing a mixture of a carbon source gas and a second protecting gas, S5. In the present embodiment, the carbon source gas can be hydrocarbon, such as acetylene or ethylene while the protecting gas can be inert gas or nitrogen. Particularly, acetylene is chosen as the carbon source gas by virtue of its low decomposition temperature and nitrogen is used as the second protecting gas. In addition, the first protecting gas and the second protecting gas can be the same gas. In step 5, S5, once the mixture of carbon source gas and second protecting gas is introduced into the reaction device, the carbon nanotubes are grown in a temperature range from about 400-800 C for about 5-30 minutes. The reaction device is then cooled down and the metal substrate is taken out from the reaction device.

Referring to FIG. 2 and FIG. 3, the carbon nanotubes fabricated by the method, in accordance with the present embodiment, are disorderly arranged on the metal substrate. One end of each carbon nanotube is connected with the surface of the metal substrate. In addition, a diameter of each carbon nanotube is in a range from about 5 nm to 20 nm.

In conclusion, the carbon nanotubes fabricated by the method of the present embodiment can be directly formed on the metal substrate. There is no need to coat a catalyst layer on the metal substrate in advance for growth of the carbon nanotubes. Therefore, the manufacturing procedure is simplified and the manufacturing cost is decreased that is suitable for mass production.

Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.

It is also to be understood that above description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps. 

1. A method for making carbon nanotubes, the method comprising the following steps: providing a metal substrate; polishing a surface of the metal substrate; putting the polished metal substrate into a reaction device; introducing a first protecting gas while heating the environment inside of the reaction device to about 400 to 800 degrees; and introducing a mixture of a carbon source gas and a second protecting gas, whereby the carbon nanotubes are grown directly on the polished metal substrate.
 2. The method as claimed in claim 1, wherein the step of polishing the surface of the metal substrate comprising the following steps: rubbing the surface of the metal substrate along a first direction; then rubbing the surface of the metal substrate along a second direction; and then repeatedly rubbing the surface of the metal substrate along the first direction.
 3. The method as claimed in claim 2, wherein the surface of the metal substrate is first rubbed along the first direction for about 3 to 5 minutes, then is rubbed along the second direction for about 5 to 8 minutes, and then is rubbed along the first direction for about 10 to 15 minutes.
 4. The method as claimed in claim 2, wherein the surface of the metal substrate is first rubbed along the first direction with an abrasive paper of about 600 to 800 grit, then is rubbed along the second direction with an abrasive paper of about 1000 to 1300 grit, and then is rubbed along the first direction with an abrasive paper of about 1500 to 2000 grit.
 5. The method as claimed in claim 2, wherein an angle between the first direction and the second direction is larger than 0 degrees and less than or equal to 90 degrees.
 6. The method as claimed in claim 1, further comprising a step of removing powder generated from polishing the surface of the metal substrate.
 7. The method as claimed in claim 6, wherein the powder is removed by the application of air flow.
 8. The method as claimed in claim 1, wherein the metal substrate comprises of copper.
 9. The method as claimed in claim 1, wherein the metal substrate is a rectangular.
 10. The method as claimed in claim 1, wherein a thickness of the metal substrate is in a range from about 0.5 centimeters to about 5 centimeters.
 11. The method as claimed in claim 1, wherein the reaction device is a box furnace or a tube furnace.
 12. The method as claimed in claim 1, wherein the first or second protecting gas is inert gas or nitrogen.
 13. The method as claimed in claim 1, wherein the carbon source gas is acetylene or ethylene.
 14. The method as claimed in claim 1, wherein a growth time for the carbon nanotubes is in a range from about 5 minutes to 30 minutes.
 15. The method as claimed in claim 1, wherein the first protecting gas and the second protecting gas comprise of the same gas. 